Dark Energy Paints the Void

by Paul Gilster on August 27, 2007

A vast, empty region in Eridanus may be giving us hints about the operation of dark energy in the distant universe. The region shows up on the Wilkinson Microwave Anisotropy Probe’s map of the cosmic microwave background (CMB) radiation. The remnant of the Big Bang, the faint radio waves of the CMB provide the earliest picture we have of the cosmos. What the WMAP displayed to us was a view of its structure at a time just a few hundred thousand years after the Big Bang.

The Eridanus region stands out on the WMAP data because it’s slightly colder, and I do mean ‘slightly’ — we’re talking about temperature differences in the area of millionths of a degree. Two possibilities thus arise: The cold spot could be intrinsic to the CMB itself, a structural anomaly in the early universe. Or it could indicate something through which the CMB radiation had to pass on its way to our detectors. Now a study using data from the National Radio Astronomy Observatory VLA Sky Survey offers a possible confirmation of the latter.

For the Eridanus region shows a marked drop in the number of galaxies that would be expected there. Says Lawrence Rudnick (University of Minnesota):

“Although our surprising results need independent confirmation, the slightly lower temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth.”

And that’s abnormal indeed. Yes, the universe is known to feature voids largely empty of matter, but none on this scale, nor does the magnitude of this ‘hole’ gibe with computer simulations of the large-scale structure of the universe. And the observational effect being examined may involve dark energy. The lack of matter creates lower temperatures in the CMB, the team theorizes, because CMB photons that pass through the void before reaching Earth should have less energy than those that pass through space filled with a normal distribution of matter.

Here’s the gist of what the team is arguing. With the paper not yet available online, I’ll have to work solely off the news release:

In a simple expansion of the universe, without dark energy, photons approaching a large mass — such as a supercluster of galaxies — pick up energy from its gravity. As they pull away, the gravity saps their energy, and they wind up with the same energy as when they started.

But photons passing through matter-rich space when dark energy became dominant don’t fall back to their original energy level. Dark energy counteracts the influence of gravity and so the large masses don’t sap as much energy from the photons as they pull away. Thus, these photons arrive at Earth with a slightly higher energy, or temperature, than they would in a dark energy-free Universe.

Conversely, photons passing through a large void experience a loss of energy.

But this work, as Rudnick said above, needs confirmation. As team member Liliya Williams (also at the University of Minnesota) emphasizes, “What we’ve found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the Universe.” Accounting for the size of this void and its relationship to the rest of the WMAP data will doubtless yield new surprises. Let’s hope it also has more to tell us about dark energy itself. The paper is slated for publication in The Astrophysical Journal; full references when they become available.

There’s problems aplenty for simplistic inflationary cosmology, but the Big Bang is more like an observational fact than a theory. We know from the CMB and measurements of gas temperatures in the past that the Universe passed through a period of high temperature and high density – the Big Bang – but what caused it and what preceded it is entirely up for grabs theoretically.

A paper came out recently that suggests the Universe is actually compact – specifically a 3-torus – and its fundamental scale is only a few Giga-parsecs. Now that’d be really making people scratch their heads because such a small Universe is inexplicable using inflation and totally negates speculations on “infinite Universes” etc etc. Cold spots might be easier to explain in such a scenario?

It was my understanding that ideas like inflation came about in order to explain non-uniformity such as this recently discovered void. That the big bang, in and of itself, should produce a universe that is more uniform. I think some of the “small universe” ideas also have problems explaining such a void.

Inflation is supposed to explain the observed isotropy of the CMB – the fact that it is so alike across virtually all of the sky. Anisotropies have to be explained by other means. By itself the Big Bang doesn’t predict either way, but the observed flatness of the CMB constrains all theories. The void is a puzzle, but only for inflation.

Abstract: CONTEXT: Cosmic voids are observed in the distribution of galaxies and, to some extent, in the dark matter distribution. If these distributions have fractal geometry, it must be reflected in the geometry of voids; in particular, we expect scaling sizes of voids. However, this scaling is not well demonstrated in galaxy surveys yet.

AIMS: Our objective is to understand the geometry of cosmic voids in relation to a fractal structure of matter. We intend to distinguish monofractal voids from multifractal voids, regarding their scaling properties. We plan to analyse voids in the distributions of mass concentrations (halos) in a multifractal and their relation to galaxy voids.

METHODS: We make a statistical analysis of point distributions based on the void probability function and correlation functions. We assume that voids are spherical and devise a simple spherical void finder. For continuous mass distributions, we employ the methods of fractal geometry. We confirm the analytical predictions with numerical simulations. Smoothed mass distributions are suitable for the method of excursion sets.

RESULTS: Voids are very nonlinear and non-perturbative structures. Voids reflect the fractal geometry of the matter distribution but not always directly: scaling sizes of voids imply fractal geometry, but fractal voids may have a complicated geometry and may not have scaling sizes. Proper multifractal voids are of this type. A natural multifractal biasing model implies that the voids in the galaxy distribution inherit the same complicated geometry.

CONCLUSIONS: Current galaxy surveys as well as cosmological N-body simulations indicate that cosmic voids are proper multifractal voids. This implies the presence in the voids of galaxies or, at least, small dark matter halos.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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